Amniote traits and terrestriality in an early amphibian

Amniote traits and terrestriality in an early amphibian | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Biological Sciences - Article Amniote traits and terrestriality in an early amphibian Michael Coates, Benjamin Otoo, Abigail Caron, Kristen Tietjen, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-9013156/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract The origin of terrestrial tetrapods is linked to the evolutionary split between amniotes, which occupied dryer upland environments, and stem tetrapods and lissamphibians which diversified into wetlands1-3. Skeletal evidence of early lissamphibian habits includes broad-based skulls with sensory lateral lines, gill bars, pharyngeal teeth, and weakly ossified vertebrae4-6. In contrast, early amniotes generally exhibit narrow-based skulls, well ossified postcrania, and limbs with evidence of claws7,8 implying presence of keratinized, resilient, watertight skin9,10. However, histological and molecular genetic analyses of extant tetrapods find keratinization-linked processes shared throughout the crown clade, suggesting their presence in the stem-group as well11-13. But fossils have thus far provided no direct support for this possibility. Here, we report unusually complete remains of the stem-lissamphibian Eugyrinus wildi from the Upper Carboniferous (~319 million years ago) of Lancashire UK. The specimen displays clawed limbs, regionalized scaly skin, eyes adapted for subaerial vision, and gut contents including terrestrial prey. In addition to supporting the stem-tetrapod keratinization hypothesis11-13, these new data imply that adults of the last common ancestors of living tetrapods were terrestrially adapted even though early life history stages remained tied to aquatic conditions4,6,14. Further, these findings emphasize the derived condition of modern Lissamphibia, suggest emergent behaviours of early temnospondyls in coal swamp habitats, and question the value of early claw-marked fossil trackways as definitive evidence of crown group amniotes and, therefore, as calibrations in tetrapod phylogeny15. Biological sciences/Evolution/Palaeontology Biological sciences/Evolution/Phylogenetics Full Text Additional Declarations There is NO Competing Interest. Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-9013156","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Biological Sciences - Article","associatedPublications":[],"authors":[{"id":601166798,"identity":"39a45680-55f4-44bc-b3ca-896bc73af6ac","order_by":0,"name":"Michael Coates","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA2klEQVRIie3QMQrCMACF4YjQLkHXTO0JhEqgeByL0C7VRRCHGgqCLuruLerinBBolrj3EA6CIJ3EKLo4JHQTzD8l8D4CAcBm++E8ACCg70tg3qspbk6iXJFPetJbc3a9zElSiBOjdbbwgLs8Ih0JZTxCVPJxISdDtikFBrCc6UkFA8RWdFyoAwdOGeUoDU0E1+xOkuBF7or4ZyMJEcvbwxdprTL1CjQQGccDWfL+XqYB2+4odmA8HWiJ4LyaZ8TvCIkv9Y14XZcfKh35Tv1Ak/kz0hTYbDbbH/QAa6NQdr364l0AAAAASUVORK5CYII=","orcid":"https://orcid.org/0000-0003-2843-1075","institution":"University of Chicago","correspondingAuthor":true,"prefix":"","firstName":"Michael","middleName":"","lastName":"Coates","suffix":""},{"id":601166799,"identity":"53e64843-637f-4794-8820-36e32d5b373c","order_by":1,"name":"Benjamin Otoo","email":"","orcid":"","institution":"Earth Sciences Department, Carleton University","correspondingAuthor":false,"prefix":"","firstName":"Benjamin","middleName":"","lastName":"Otoo","suffix":""},{"id":601166800,"identity":"d5560009-7302-4f5f-b770-8381194cb989","order_by":2,"name":"Abigail Caron","email":"","orcid":"https://orcid.org/0000-0002-0906-8218","institution":"University of Chicago","correspondingAuthor":false,"prefix":"","firstName":"Abigail","middleName":"","lastName":"Caron","suffix":""},{"id":601166801,"identity":"f463cb29-8400-4f34-9a58-29688a1e774b","order_by":3,"name":"Kristen Tietjen","email":"","orcid":"","institution":"University of Chicago","correspondingAuthor":false,"prefix":"","firstName":"Kristen","middleName":"","lastName":"Tietjen","suffix":""},{"id":601166802,"identity":"27b674e8-02cb-45f2-b4de-57794a9572b5","order_by":4,"name":"Marcello Ruta","email":"","orcid":"https://orcid.org/0000-0002-6151-0704","institution":"University of Lincoln","correspondingAuthor":false,"prefix":"","firstName":"Marcello","middleName":"","lastName":"Ruta","suffix":""}],"badges":[],"createdAt":"2026-03-02 18:55:30","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-9013156/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-9013156/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":106404648,"identity":"f603ce29-9386-48df-86fd-b7100137a94e","added_by":"auto","created_at":"2026-04-08 09:16:27","extension":"pdf","order_by":1,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":5776086,"visible":true,"origin":"","legend":"Article File","description":"","filename":"Coatesetal.2026.pdf","url":"https://assets-eu.researchsquare.com/files/rs-9013156/v1_covered_735d3716-03a3-4f09-a70f-04f086f28bdd.pdf"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Amniote traits and terrestriality in an early amphibian","fulltext":[],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":false,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":true,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":true,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-9013156/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-9013156/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"The origin of terrestrial tetrapods is linked to the evolutionary split between amniotes, which occupied dryer upland environments, and stem tetrapods and lissamphibians which diversified into wetlands1-3. Skeletal evidence of early lissamphibian habits includes broad-based skulls with sensory lateral lines, gill bars, pharyngeal teeth, and weakly ossified vertebrae4-6. In contrast, early amniotes generally exhibit narrow-based skulls, well ossified postcrania, and limbs with evidence of claws7,8 implying presence of keratinized, resilient, watertight skin9,10. However, histological and molecular genetic analyses of extant tetrapods find keratinization-linked processes shared throughout the crown clade, suggesting their presence in the stem-group as well11-13. But fossils have thus far provided no direct support for this possibility. Here, we report unusually complete remains of the stem-lissamphibian Eugyrinus wildi from the Upper Carboniferous (~319 million years ago) of Lancashire UK. The specimen displays clawed limbs, regionalized scaly skin, eyes adapted for subaerial vision, and gut contents including terrestrial prey. In addition to supporting the stem-tetrapod keratinization hypothesis11-13, these new data imply that adults of the last common ancestors of living tetrapods were terrestrially adapted even though early life history stages remained tied to aquatic conditions4,6,14. 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